1. Field of the Invention
This invention relates to an improved chemical milling solution and method for milling, etching or pickling metal products therewith. More particularly, the invention relates to a bath composition and method for milling or pickling titanium workpieces, such as forgings or the like.
2. Technology Review
As used herein, the term "milling" shall mean the selective and controlled removal (or corrosion) of metal (or metal oxides) from a part or object by chemical milling, etching and/or pickling. Most milling procedures form metal product of a desired thickness and/or configuration by removing metal from a treated workpiece thereby imparting greater weight savings to aerospace parts or the like. Milling operations are typically performed after a particular metal part has been formed by casting, forging, extrusion or rolling; and heat treated. Milling is also used to make shapes which cannot otherwise be machined by conventional chipmaking techniques, or which can only be machined by known methods at unreasonably high cost. For many parts, masking of certain areas is done to prevent their exposure to a corrosive milling solution.
As used for the description of this invention, "milling" shall also include metal etching, the controlled removal of metal for dimensional and shape control, and metal cleaning or pickling, i.e., the removal of embrittled oxidized surfaces. For titanium alloys, oxidized surfaces are sometimes referred to as alpha-case. Such surfaces may result from exposure to elevated temperatures during the manufacturing process, i.e., casting, rolling, extrusion, forging or the like.
Any chemically dissolvable metal may be subjected to treatment by the aforementioned milling practices. Alloys of aluminum, beryllium, magnesium, titanium and various steels are the most commonly milled metal products. Refractory metals such as molybdenum, tungsten, niobium (columbium) and zirconium may also be chemically etched in the same manner. The workpieces treated by milling (i.e. chemical, etching and/or pickling) need not be limited in size provided a large enough bath of milling solution can be maintained. Milled parts may be cast, forged, extruded or rolled. Their end shapes may be flat, tubular or in any of the complex configurations required by today's manufacturers of aerospace, automotive and other parts.
The first chemical milling practices are believed to have occurred around 2500 B.C., when ancient Egyptians used citric acid to etch copper jewelry. Current industrial milling practices can be traced back to the methods set forth in Sanz U.S. Pat. No. 2,739,047. Numerous evolutions to milling solutions have occurred since modern milling procedures were patented over 35 years ago. Many of these solution developments depended on the particular metal alloy being milled.
For titanium and titanium-based alloys, Chen U.S. Pat. No. 4,900,398 claims a milling method which uses an aqueous solution consisting essentially of 1-5% hydrofluoric acid, about 1.5-4% chlorate ion and up to about 20% of an acid selected from the group consisting of H.sub.2 SO.sub.4, HCl and HNO.sub.3. Although the latter acid additive is claimed as being an optional component, the only example solution from this reference requires 20 ml/l of 98% H.sub.2 SO.sub.4 (or 3.6% by weight). More preferred embodiments claim about 4% sulfuric acid whereas the present milling bath is substantially sulfuric acid-free.
Many current practices for chemically milling, etching and pickling titanium workpieces employ chromic or nitric acid in a hydrofluoric acid-based bath. Hexavalent chrome is a suspect carcinogen, however, and nitric acid releases visible fumes of toxic NO.sub.x during standard milling operations. Production facilities have been under increasing regulatory pressure to reduce or eliminate such emissions from the workplace.